Hydrogenated amorphous silicon has been demonstrated as having favorable photoconductive properties, promising a viable alternative to crystalline materials such as single crystal silicon and germanium. Produced typically in a thin film form, amorphous silicon provides substantial material savings over its crystalline counterparts. The existing impediment to its widespread use is a low device efficiency relative to other materials. Although the material displays a favorable quantum efficiency of photogenerated charge carriers, other fundamental electrical properties of the semiconductor such as mobility, lifetime and diffusion length of carriers, for example, limit the efficiency of an amorphous silicon device. The resultant effect upon a device such as a solar cell is that the effective collection of photo-generated charge carriers is limited to the barrier region or the non-junction region is not electronically neutral, thereby impeding the transport of carriers generated in the barrier or depletion region.
The present invention deals with these deficiencies by selectively altering the amorphous silicon with an extrinsic dopant which both extends the effective field region of charge collection substantially throughout the amorphous silicon layer and concurrently improves the electrical characteristics of the non-junction or bulk region of the device. The dopant comprises an ionizable material such as antimony, for example, which is thermo-electrically diffused into the amorphous silicon layer during the sputter deposition of the silicon film.
The two principal methods of producing hydrogenated amorphous silicon are the glow discharge decomposition of silane and reactive sputtering in a plasma consisting of a mixture of argon and hydrogen. In either case the material has been doped N or P-type by adding to the discharge an amount of phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6); respectively. Solare cell structures have been fabricated from these materials utilizing abrupt junctions formed by gas phase doping. Such structures include for example Schottky barriers, p-i-n junctions and hetero-junction configurations. The Schottky barrier structure is a multilayer configuration consisting of a metallic substrate, a 500A heavily phosphine doped a-Si layer (n.sup.30 ), an intrinsic amorphous silicon layer, and a high work function semi-transparent metal contact. The thin n.sup.+ amorphous silicon layer, obtained by doping from a discharge containing PH.sub.3, is used to form the ohmic contact to the intrinsic amorphous silicon layer.